Hello,
I'm happy to announce WebGL playground, an online tool for playing with WebGL. I hope it will be useful for many of the forum readers!

http://webglplayground.net

It's work in progress. I'm open to suggestions :)

some more details:
WebGL playground is a straightforward idea: type in
your WebGL script and see the results. But the cool
part is that the editor and the results are just on
the same page and that you get a handful of features
that make your life easier.

The editor lets you work on the JavaScript code and
the GLSL vertex/fragment shaders (if you have any)
at the same time in a convenient way. Everything is
organized, formatted and highlighted properly, just
as you would like!